APSC 380 : I NTRODUCTION TO M ICROCOMPUTERS
1998/99 W INTER S ESSION T ERM 2
Power Control Circuits
Most high-power actuators operate from AC power supplies. This lecture briefly describes devices and circuits that
are used to control AC power sources.
After this lecture you should be able to analyze the operation of, and draw schematics for power control circuits using
SCRs, diacs and triacs.
Introduction
Although SCRs are typically used with AC voltages, they can also be used to control DC supplies beWe have seen how bipolar transistors and MOSFETs cause they are cheaper and more efficient than trancan be used to control actuators that operate on DC sistors. In this case the SCR can be turned off by usvoltages. The devices described in this lecture, SCRs ing a switch to short across (“commutate”) the SCR.
and Triacs, are used to control actuators (e.g. motors)
that operate on AC power and to convert AC to DC
for high-power applications.
Half- and Full-Wave Rectifiers
SCRs
Rectifier circuits uses diodes to convert AC into timeControl of high-power AC equipment is typically
varying DC. A half-wave rectifier conducts only durdone using devices called silicon controlled rectifiers
ing half of the cycle, while a full-wave rectifier con(SCRs). The SCR is the most common of a class of
ducts during the whole cycle. If a center-tapped
devices called thyristors.
transformer is available two diodes are sufficient,
otherwise four diodes are required.
An SCR is similar to a diode in that it conducts
current only in one direction (anode to cathode).
However, the SCR has an additional gate terminal.
Like a bipolar transistor, a small current must flow
from the gate to the cathode for the SCR to conduct
(“fire”). However, unlike a transistor, the SCR will
continue to conduct as long as there is current in the
anode-cathode circuit even if the gate current is removed. The SCR will “turn off” when the current
in the anode-cathode circuit drops below a minimum
value.
Some typical SCR specifications are the maximum
on- and off-state voltages and currents, minimum
turn-on and turn-off gate current and turn-on time.
SCRs are available in ratings up to several thousand
volts and amps.
lec12.tex
SCR for Voltage Control
Instead of using diodes in the above circuit, we can
use SCRs. By varying the fraction of time into the
cycle before the SCR is turned on, we can vary the
fraction of the AC cycle which appears on the output. This in turn affects the average value of the output. For many types of devices (e.g. lights, heating
elements, some types of motors) a time-varying DC
waveform is sufficient. For other applications the DC
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will have to be filtered.
The solid-state relay works similarly to the optoisolator that was described earlier. The control side
uses an LED to turn a light-operated SCR or transistor (for DC) or triac (for AC) on and off. These
solid-state relays are available as encapsulated modules with guaranteed minimum isolation between the
input and controlled terminals.
In this case the SCRs will turn off by themselves
when the gate current is turned off and the AC voltage reaches zero (twice per cycle).
Triacs
A triac is a device that behaves like two SCRs connected in parallel anode-to-cathode with one common gate. This arrangement allows current flow in
both directions through the same device. By controlling the point in the waveform when the triac is
turned on, it can also be used to control the average
power delivered to the load.
A triac is often used with another device called
a diac which conducts current (in either direction)
when the voltage across it exceeds a certain threshold. By using an RC circuit to vary the phase of the
voltage applied to the diac, it is possible to control
the turn-on time of the triac within the cycle. This is
how common light dimmers work.
Transistor Switching for Generating
AC
By using transistors to switch a DC supply on and off
it is possible to create a square wave. If this square
wave drives a current through the primary of a transformer then we can obtain a higher (or lower) AC
voltage at the secondary by using a transformer with
an appropriate turns ratio. This is how “power inverters” are used to drive AC-powered devices from
batteries.
We can also control the frequency of the AC waveform thus control the speeds of certain types of
(“synchronous”) AC motors.
Solid state Relays
3-phase circuits
A “solid-state relay” is a circuit that has the same
purpose as a magnetically-operated relay. It allows a
low-power circuit to turn a high-power circuit on and
off.
The relay isolates the control circuit (usually lowvoltage electronics) from the controlled circuit (usually high-voltage power devices) and prevents voltages from being transferred to the control circuits
and causing damage.
While low-power loads (less than about 15 A at 120
VAC=1800kVA) can use the types of single-phase
power control circuits described above, most highpower (e.g. more than 25 A at 240 VAC = 6 kVA)
AC devices operate on 3-phase power. The principles and components required are the same but the
control electronics must switch three phases instead
of one.
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